Normally tacky pressure sensitive adhesive (PSA) compositions suitable for use in the manufacture of tapes and labels, for example, must possess a proper balance of the properties of adhesion, cohesion, stretchiness, and elasticity. Here, adhesion refers both to that property of immediate adhesion to a surface which is often termed "quick tack" or "quick stick" and to the "peel strength" or bond strength which develops when more complete surface contact is effected via the application of pressure. Cohesion refers to the property of "shear strength" or resistance of the applied PSA to failure when subjected to shearing forces. For PSA compositions derived from acrylic copolymers, it is not only possible to achieve this requisite four-fold balance of properties but also to benefit from the inherent, desirable characteristics of transparency and resistance to oxidation. Acrylic copolymeric PSAs also offer versatile processability and may be coated from solution (or emulsion) or via hot melt techniques.
While acrylic copolymeric PSA compositions (such as those described by Ulrich in U.S. Pat. No. Re. 24,906) have been found to be more than adequate for many adhesive applications and have therefore garnered tremendous continuing commercial success, there are many applications which require greater shear strength than can be provided by these relatively soft materials. For example, disposable diaper tapes, permanent labels, and packaging tapes must be able to adhere well and to retain their adhesive bond formation under exposure to high shear forces. PSAs for medical tape applications also require significant internal strength such that peeling does not result in cohesive failure, leaving residual adhesive on the skin. Attempts have been made to enhance the shear strength of acrylic PSAs, but the quick tack and peel values of the resultant compositions have generally diminished as the shear values have increased. This is due to the fact that the aforementioned balance of PSA properties is indeed a balance of interdependent properties, and it is extremely difficult to improve the internal or cohesive strength without also upsetting the other properties and destroying the overall pressure-sensitive nature of the adhesive system.
Thus, various ways of reinforcing, i.e., improving the shear strength, of acrylic copolymeric PSAs are known but each has its disadvantages. The molecular weight of the copolymer can be increased to improve internal strength, but this generally reduces tack and peel strength and produces a composition with poor processability. The polar monomer, e.g., acrylic acid, content of the copolymer can also be increased but, with such increase, there is generally a loss of adhesive tack or quick stick, as well as peel strength, due to the decreased compliance of the PSA. Crosslinking may also be utilized to enhance the cohesive strength of an acrylic PSA composition, but it often requires expensive equipment and/or additional process steps. Chemical crosslinking often causes a shortening of pot life. The crosslinked adhesive composition may also exhibit loss of tack and peel strength as well as reduced processability. Moreover, difficulties in achieving reproducible levels of crosslinking are common, and some systems may involve crosslinking reactions which continue for long periods of time after manufacture, resulting in noticeable changes in PSA properties with time. There has thus been long-standing interest in finding a simple method for improving the cohesive strength of an acrylic copolymeric PSA without crosslinking.
U.S. Pat. No. 4,554,324 discloses that crosslinking may be avoided via the grafting of acrylic copolymers with reinforcing, thermoplastic polymeric moieties, e.g., polystyrene, to obtain enhanced shear strength without sacrificing processability. However, this grafting method is not a simple means of reinforcement of commercially available acrylic PSAs because it requires sophisticated process steps in order to effect the chemical modification of the acrylic copolymer. While a simple blending of acrylic adhesive and reinforcing material may produce a reinforced adhesive composition, that is not always the case. U.S. Pat. No. 4,500,683 teaches that no marked improvement in cohesive strength is achieved via mere blending of an acrylic PSA with a high Tg (greater than 273.degree. K.) homopolymer (or copolymer) derived from ethylenically unsaturated monomer(s) such as styrene or vinyl acetate. This patent further states that such blending produces a significant, undesirable drop in adhesive strength and that the desired reinforcement with retention of balanced PSA properties can be achieved only by homo- or co-polymerizing one or more ethylenically unsaturated monomers in the presence of the acrylic polymer to form a composition containing an addition-polymerization polymer.
Blending processes have been utilized successfully for the select purpose of reinforcement of acrylic copolymers of low molecular weight. U.S. Pat. Nos. 4,337,325, 4,370,380, and 4,423,182 describe hot melt coatable acrylic PSA compositions exhibiting enhanced shear strength and good adhesive properties (i.e., good tack and peel). The first two patents teach the use of vinyl lactam polymers or copolymers as blended reinforcing agents, and the third patent describes ionomeric PSAs which are reinforced via the addition of at least one miscible metal salt of an o-methoxy aryl acid. In each case, it was possible to achieve a balance of adhesive and cohesive PSA properties through the use of low molecular weight (hot melt coatable) acrylic copolymers which, by virtue of their molecular weight, possess very good tack and adhesive properties but are of insufficient cohesive strength to be useful alone as PSAs. Such tacky "non-PSA" materials can tolerate the substantial losses in tack and adhesive or peel strength which, as noted above, generally accompany an increase in the cohesive or shear strength of a PSA composition. However, acrylic copolymers of sufficient molecular weight, and, thereby, cohesive strength, to function alone as PSAs are inherently less tacky and much more limited in their ability to tolerate such losses.
Transparent, water-soluble PSA compositions are described in U.S. Pat. No. 3,657,396 which, again, teaches that reinforcement of highly tacky polymers can be achieved via blending. Rather than dealing with acrylic polymers, however, this patent concerns poly(vinylmethyl ether)-based formulations which are blended with cellulosic polymers, water-soluble phenolic resins, water-soluble polyvinyl compounds such as poly(vinyl alcohol) or poly(vinyl pyrrolidone), water-soluble acrylic homopolymers or copolymers, vinyl methyl ether-maleic anhydride copolymers, or carboxylic-modified poly(vinyl acetate).
Polymers of 2-oxazolines are known and, often in the form of blends with compatible polymers, have proven utilities as hot melt and/or water dispersible adhesives and as adhesion enhancers. For example, U.S. Pat. Nos. 4,436,867 and 4,522,967 teach the use of polyethyloxazoline in water dispersible, non-pressure sensitive adhesive compositions. The former patent describes creping adhesives consisting of an aqueous mixture of polyethyloxazoline and a thermoplastic polymer such as poly(vinyl alcohol), poly(vinyl pyrrolidone), or ethylene-vinyl acetate copolymer. The latter patent discloses a water dispersible, heat activatable adhesive composed of polyethyloxazoline, a water dispersible plasticizer (such as an ethylene-vinyl acetate copolymer, a water dispersible acrylic polymer, or a water dispersible vinyl acetate-acrylic copolymer), and an anti-blocking agent such as polyethylene glycol or polypropylene glycol. U.S. Pat. Nos. 4,474,928 and 4,532,187 are also adhesive-related and concern the addition of poly-2-oxazoline to a polyolefin or an ethylene-carboxylic acid copolymer, etc., to enhance adhesion to a substrate. It is noted that the blend can be used as a hot melt or solvent-based (non-pressure sensitive) adhesive. Oxazoline polymers have also been utilized as viscosity modifiers, components of semipermeable membranes, etc. However, the use of poly-2-oxazolines in pressure sensitive adhesive compositions has, to our knowledge, not been described.